Location of new foundation with old one

[Eabrar]

Hi
We are carrying out some modification in our plant for which structure is required. The location that we have selected is near any existing struture and a column.

Can anyone tell me what is the safe distance for the new foundation from existing one.
Existing foundation is have piles and for new foundation new 24" piles will be drilled which is approximately 1/2 meter away from existing foundation of column.

Abrar

 

[SlideRuleEra]

The key factor is how far apart the piles are located. One "rule of thumb" is that piles should be no closer than
3 times their "diameter". If the piles in the existing structure are 24" or less then a pile spacing of 6 feet center to center (minimum) would be a starting point. You probably need a geotechnical engineer involved in the project to advise on a final decision.

Keep in mind that the distance between the existing foundation cap and the new piles is not too important - the pile SPACING is what is important.

 

[PEinc]

You also need to consider the elevations of the two pile caps. Is the bottom of the new pile cap at a lower elevation than the bottom of the existing pile cap? You don't want to dig below the existing pile cap and lose the dirt from under the existing pile cap.

You also need to be careful that excavation or drilling the new piles does not remove the lateral support of the existing piles. If you take away the dirt from one or more sides of the existing piles, you will cause an unsupported length for which the existing piles were not designed. This would reduce the pile capacity and could cause buckling.

Will you be drilling below the ground water table? Will this cause running soil and loss of ground in the drill hole near the existing piles?

 

[Eabrar]

Hi
Thanks for your reply.

Actually their is no pile under column foundation but their are piles under existing structure foundation. Pile size of existing foundation is 12", 15 m depth and the pile size for new foundation is 24” with 20 m depth. Minimum centre to centre distance between existing pile and new pile is 4.5 ft.

Depth of pile cap of existing structure foundation is above 1.6 meter below gorund level and depth of pile cap of new foundation is 2 m below ground level.

We are using reverse rotary rig for drilling of piles in which water is continuously circulated and water level is maintained to prevent caving of borehole.

1. Is there is any effect of water on existing foundations strength?
2. Is it safe to drill pile near column foundation during plant operation?
3. Is minimum distance between piles is safe?

Best regards

Abrar

 

[PEinc]

I don't think WATER maintains the stability of a bore hole. If it did, it would be easy to dig deep vertical holes at the beach. Don't you need some type of drilling fluid or slurry that is heavier than water?

 

[SlideRuleEra]

Abrar - Sounds like you have a very bad situation. The combination of pile spacing that is too tight, drilling below the existing pile tips, circulating water in the holes, and working during plant operations is not good. Again, I strongly suggest that you get a geotechnical engineer involved in this project before proceeding.

On an industrial job I was involved in a man was killed (swallowed alive) by sudden earth subsidence during drilling operations. Your conditions sound like a recipe for BIG trouble.

 

[BigH]

Eabrar: Who is your project's geotech? If you don't have one, why didn't you? He'd be in a perfect situation to advise you of your concerns. Now, during the installation of the piles, etc., it is a bit late - delays, etc. to keep in mind. One other thing - what kind of soils are you in? If you are in cohesive soils, water might work as the ground is more or less stable without support.

 

[boo1]

Could you drive sheet piles to seperate the two areas?